The growth of large essentially single crystal ingots or macrocrystals of various inorganic salts for use as optical bodies has been the focus of much attention over the past decade. Among the salts especially suited for use as lenses over a wide range of wavelengths of radiation, are the alkaline earth metal halides. Most preferred are the fluorides of barium and calcium.
This invention is particularly directed to a barium fluoride (BaF.sub.2) macrocrystal ingot because of its especially frangible nature. Ever since interest was sparked in BaF.sub.2 optical bodies, it has been well-known that a relatively large ingot of BaF.sub.2 will often shatter when being cut, sometimes with explosive force. Relatively small ingots, less than about 5 cms. in diameter, are less likely to shatter. The manner in which the ingot is cut appears to be irrelevant, as does the speed of cutting and other related factors. Despite taking ever conceivable precaution during the growth of an ingot of barium fluoride, there was no way of predicting whether or not the ingot could be cut without shattering, and there was no clue as to why an ingot shattered. Not unexpectedly, whether or not a melt-grown ingot of BaF.sub.2 was malleable under sufficiently high temperature and pressure, was equally unpredictable. The enormous expense of growing a BaF.sub.2 crystal ingot without being able to receive any assurance that the ingot will not shatter prematurely, fueled the efforts which resulted in the discovery that a concentration of strontium or calcium above about 10 ppm by weight was determinative of the malleability of the ingot. This invention is directed to a malleable BaF.sub.2 ingot and to a process for obtaining an essentially strontium-free and calcium-free barium fluoride salt from which the ingot may be grown.
A macrocrystal or ingot of the present invention may be prepared by any conventional crystal growing technique for melt-growing an essentially single crystal ingot, such as the Stockbarger-Bridgman procedure (U.S. Pat. Nos. 2,498,186 and 2,149,076).
Prior art BaF.sub.2 essentially single crystal ingots cannot be press-forged. They are not malleable. They shatter. Though a very large number of BaF.sub.2 ingots have been melt-grown from substantially pure barium fluoride obtained from reagent grade barium salts, such as barium carbonate by reaction with hydrofluoric acid, the ingots were contaminated with an unacceptably high concentration of calcium or strontium or both carried over from the barium carbonate. For example, a typical reagent grade barium carbonate powder contains up to 0.30 percent (3000 ppm) strontium; typical reagent grade barium nitrate crystals contain up to 0.050% (500 ppm*) strontium. The commercially available pure barium nitrate contains up to 500 ppm each of strontium and calcium (AR grade Ba(NO.sub.3).sub.2, Mallinckrodt 378a). Ingots melt-grown from such available starting materials therefore contained at least a strontium impurity in a concentration far higher than 10 ppm. Despite conventional purification of these commercially available crystalline powders, as for example, by several recrystallizations from solution, the purified salts failed to produce a predictably non-shattering ingot of BaF.sub.2. It is conceivable that, with a large enough number of recrystallizations under suitable conditions, sufficiently contaminant-free BaF.sub.2 cyrstals may be obtaind. The process of this invention is a practical alternative. FNT *see J. T. Baker Laboratory Chemicals and Products Catalog
It is recognized that crystalline barium fluoride powders essentially free of strontium may have been prepared for a specific purpose, but we are unaware of such ultrapure BaF.sub.2 having been prepared by a wet process utilizing a concentrated nitric acid medium.
The difficulty of successfully separating barium salts from those of calcium and strontium is well-recognized even in known qualitative analytical procedures. For example, it is known that "The most satisfactory method of separating Ba from Ca and Sr is a double precipitation of barium chromate from a slightly acid solution containing ammonium acetate." (P. 298, Advanced Qualitative Analysis, by H. H. Willard and H. Diehl, Van Nostrand Co. Inc., N.Y. 1943). There is no indication however, as to how effectively trace amounts of Ca or Sr may be excluded from the barium chromate.
It is known that analytically pure BaCl.sub.2 and Ba(NO.sub.3).sub.2 can be decontaminated from Ca, Sr, Fe and other impurities with sequestering agents such as EDTA (ethylenediaminetetraacetic acid) solution (see "Purification of barium, strontium and calcium carbonates" by Nakhodnova A. P. et al., Zh. Prikl. Khim. 39 (3), 498-501, 1966, Russ.); by treating with ZnCl.sub.2 (see "Purification of barium chloride solutions" by Akhmetov, T. G. et al., U.S.S.R. Pat. No. 262,104); and, by various ion exchange means.
More particularly a method is known for preparation of pure barium carbonate comprising dissolving crude BaCO.sub.3 in a minimum of 18% aqueous HCl, precipitating BaCl.sub.2, dissolving precipitated BaCl.sub.2 in a minimum amount of water, redissolving and purifying the BaCl.sub.2 with carbon, reprecipitating BaCl.sub.2 redissolving in water, and precipitating BaCO.sub.3 with (NH.sub.4).sub.2 CO.sub.3 solution. The BaCO.sub.3 so obtained is washed with hot water to remove chloride ions. To obtain pure SrCO.sub.3, raw CrCO.sub.3 is dissolved in 18% HCl, neutralized with ammonia and treated with active carbon. The solution is diluted with water and chromate solution added. The solution is decanted from BaCrO.sub.3 ad some SrCrO.sub.4 and (NH.sub.4).sub.2 CO.sub.3 solution added. SrCO.sub.3 obtained is filtered, washed with water and dissolved in a minimum of 18% aqueous HCl. To this solution is added 62-65% HNO.sub.3 and Sr(NO.sub.3).sub.2 is precipitated, filtered, washed with HNO.sub.3 acid, and dissolved in a minimum amount of water. SrCO.sub.3 is precipitated with (NH.sub.4).sub.2 CO.sub.3 solution in the usual manner, filtered and washed with hot water to remove residual nitrate ions. (See "Preparation of pure barium and strontium carbonates" by Hradec Kralove, Chem. Prumysl 11, 129-31, 1961, Czech.) This use of nitric acid to precipitate strontium is contrary to the use of aqueous nitric acid to maintain strontium and calcium in solution, as is done in our invention.
As is apparent from the foregoing it is known that salts of barium, calcium and strontium are differently soluble in various liquid media. More specifically it is known that the solubilities of the nitrates of barium, calcium and strontium in aqueous solutions of nitric acid differ, and these solubilities have been measured for different concentrations of aqueous acid. The values obtained have been set forth in appropriate tables (Solubilities of Inorganic and Metal Organic Compounds, A. Seidell, Vol. I, D. Van Nostrand Co., Inc., New York, N.Y. 1940). However, the saturation tables define the behavior of a single salt in a particular medium and not the behavior of a mixture of salts. For example, these tables fail to indicate that a mixture of salts in a medium will crystallize as a coprecipitate of a solid solution of the salts. More particularly, these saturation tables are not indicative of whether or not the salts may be separated, one from another, without being coprecipitated and of course, cannot indicate the effectiveness with which a separation may be made.
It has been stated at the outset that this invention relates to a barium fluoride ingot which does not shatter when cleaved or cut, and which may be press-forged under appropriate conditions. What has not been stated is that it has been known for some time that the melt-growing of an inorganic salt ingot in a graphite crucible, whether or not it has less than the critical amounts of either calcium or strontium, is not possible if the salt is contaminated with traces of nitrate or nitrite ion. The presence of nitrate or nitrite ion in the BaF.sub.2 salt in an amount lower than the detection level with diphenylamine will cause a graphite crucible, used for growing the BaF.sub.2 ingot, to crack. It is therefore necessary, for growth in a graphite crucible, that the concentration of nitrate or nitrite ion be maintained well below 1 ppm. The problem of trace nitrate or nitrite contamination has been skirted in the past by utilizing BaCO.sub.3 as a starting material, which is converted to BaF.sub.2. Similarly CaCO.sub.3 was used to obtain CaF.sub.2. This problem does not arise in the melt growth of inorganic salts which may be effected in a crucible made of a non-graphitic material. For example, most of the ionic halides of the alkyl metals and the alkaline earth metals useful as optical bodies, may be readily obtained free of nitrate or nitrite ion, or, may be melt-grown in silica, alumina or platinum crucibles without contamination. The process of this invention comprising calcining an inorganic salt with an effective amount of ammonium fluoride and ridding the salt of nitrate or nitrite ion below the detectable level, permits using the calcined inorganic salt for growth of an ingot in a graphite crucible.